Conventionally, a drive device described in Patent literature 1 has been known as this type of drive device in the background art.
This conventional drive device has a semiconductor switch which is connected in series with an electromagnetic coil so as to control a current supplied to the coil, a capacitor, a comparator with a hysteresis function, which compares a voltage applied to the capacitor with two voltages so as to generate a signal for operating on-off of the semiconductor switch in accordance with a result of the comparison, and a charging and discharging circuit to charge and discharge the capacitor.
In the drive device configured thus, the semiconductor switch is operated to be turned on-off in accordance with the signal outputted from the comparator. In this manner, a predetermined current is supplied to the electromagnetic coil so that the coil is excited.
Since the excitation force of the electromagnetic coil is determined based on the current supplied to the electromagnetic coil, it is desirable that the current applied to the coil is measured and set at a constant value. Therefore, conventionally, the current flowing into the coil is measured by a Hall Effect element, a current sensing resistor, etc., and control is made so that the measured current becomes constant.